Assuming he had written the message on his shirt before going on the pitch, how can it be heat of the moment? He obviously intended to take his shirt off if he scored to show the message.
Indeed. I'll admit that in the greater scheme of things players taking their shirts off to celebrate is not football's biggest problem, but if the 'take your shirt off and get booked' rule wasn't in effect God knows what sort of pre-meditated rubbish we'd see every time a goal is scored. At least this way you've got to really want to get your message over.
Now if the FA can do something about players trying to take each other's shirts off in the penalty area every time there's a corner or free kick ...